FIG. 1 illustrates a conventional half bridge topology 10. As shown, the conventional topology 10 includes a pair of transistors 12 and 14 labeled Q1 and Q2. Such a topology may, for example, be implemented with enhancement mode field effect transistors (FETs). Typically, the transistors 12 and 14 are considered high side and low side switches. The source 16 of high side transistor 12 is coupled to the drain 18 of low side transistor 14 at the half bridge output. The drain 20 of high side transistor 12 is coupled to a high voltage source 22 (VMAIN), and the source 24 of low side transistor 14 is coupled to ground 26. Furthermore, the gate 28 of high side transistor 12 is coupled to a gate driver and a level shifter 30 and the gate 32 of low side transistor 14 is coupled to a gate driver 34. Gate drivers 30/34 and level shifters 30 are well-known in the art and will not be described in detail herein. However, it should be understood that this configuration enables one transistor 12 or 14 (Q1 or Q2) to be switched on and the other transistor to be switched off and vice versa during operation.
As further shown in FIG. 1, a driving voltage source 36 (VDrvr) is coupled to an input 40 of the high side gate driver and level shifter 30 as well as an input 38 of the low side gate driver 34. For this conventional design, a bootstrap capacitor 42 (CBTST) is coupled in parallel with the high side gate driver and level shifter 30 and a bootstrap diode 44 (DBTST) is coupled between the driving voltage source 36 VDrvr and the bootstrap capacitor 42 (CBTST). When using eGaN FETs for the high side and low side transistors Q1 and Q2, the configuration shown in FIG. 1 yields approximately 7.5 V, for example, across the bootstrap capacitor 42 (CBTST) if the “body diode” of low side transistor 14 (Q2) conducts. However, the voltage may vary depending on a number of factors. The voltage can be approximately 6.5 V or higher depending on the operating conditions and the type of transistor device used for low side transistor 14 (Q2). However, for this configuration, high side gate driver 30 requires some form of regulator. Conventionally, this is implemented with an internal bootstrap diode that is used to provide power to the high side transistor in the half-bridge configuration. Ideally, a Schottky diode is used because its small, it has a low forward voltage drop and no reverse recovery charge. However, it is very difficult to make a high voltage (100V) Schottky diode in an integrated circuit process and hence, PN junction diodes that have reverse recover charge are often used. Moreover, high voltage Schottky diodes can also have reverse recovery charges (QRR) that lead to losses.
To this end, when GaN FETs are used for transistors 12 and 14 (Q1 and Q2) for this type of half bridge topology, the reverse recovery charges QRR of the Schottky diodes or PN junction diodes can affect performance and induce losses in the high side transistor. Normally these losses are negligible in comparison to other power loss mechanisms in the circuit. However, the reverse recovery charge losses are proportional to frequency, and at higher operating frequencies will increase to the point where it becomes a significant portion of the converter losses. Consequently, conventional methods of bootstrapping are not efficient at high switching frequencies of operation.